1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for driving the same.
2. Description of the Related Art
Methods for driving a liquid crystal display device include a voltage averaging method (see "Ekisyo no Saisin Gijyutu (Latest Technology of Liquid Crystal)" published by Kogyo Chosakai Publishing Co., Ltd., p. 106) and a method for simultaneously selecting and driving a plurality of rows (see T. N. Ruckmongathan, Conf. Record of 1988 International Display Research Conference, p. 80 (1988); T. J. Scheffer and B. Clifton, 1992 SID Digest of Technical Papers XXIII, p. 228 (1992); and S. Ihara et al., 1992 SID Digest of Technical Papers XXIII, p. 232(1992)).
The basic principle of the voltage averaging method and the method for simultaneously selecting and driving a plurality of rows is as follows: A voltage waveform for each scanning electrode corresponding to an orthogonal matrix such as a unit matrix and a Walsh matrix is formed. Moreover, a voltage waveform for each signal electrode is formed by orthogonal transformation of display data based on the orthogonal matrix. Then, the resultant voltage waveforms are respectively applied to each scanning electrode and each signal electrode, and a voltage waveform corresponding to the difference in a voltage waveform between the scanning electrode and the signal electrode is applied to a liquid crystal panel on an intersection by intersection basis of the scanning electrodes and the signal electrodes. Thus, inverse transformation of the display data is performed on the display panel, whereby an image is displayed.
In a liquid crystal display device driven by the above-mentioned methods, a voltage waveform on each signal electrode and on each scanning electrode is distorted by reduction in sharpness or by induction at a changing point in the waveform, causing crosstalk between electrodes.
In the case where a DC voltage is continuously applied to a liquid crystal layer of the liquid crystal panel, liquid crystal will be degraded by decomposition. Accordingly, the liquid crystal panel is driven using an alternating voltage waveform of each signal electrode and each scanning electrode (this driving method is, hereinafter, referred to as an alternating driving method). In the case of the alternating driving method, crosstalk is generated significantly when a polarity of a voltage waveform changes.
Hereinafter, display on a liquid crystal panel as shown in FIG. 5 by the voltage averaging method and the alternating driving method will be described by way of example.
This liquid crystal panel has 10.times.5 dot display with signal electrodes X1 through X10 and scanning electrodes Y1 through Y5 being located perpendicular to each other. In FIG. 5, a white circle represents a pixel in an ON state, whereas a shaded circle represents a pixel in an OFF state. When the liquid crystal panel has the display as shown in FIG. 5, signals as shown in FIG. 6 are supplied to drive the liquid crystal panel.
In the liquid crystal panel, the scanning electrodes Y1 through Y5 are sequentially scanned during each frame period in synchronization with a horizontal synchronizing signal shown in (a) of FIG. 6. An alternating driving signal shown by (b) of FIG. 6 is inverted at time t1 and t2 of respective frame periods.
Each voltage waveform on the signal electrodes X1 through X10 is inverted in response to the inversion of the alternating driving signal. Referring to FIG. 5, all of the pixels on the signal electrode X4 are ON. Therefore, the voltage waveform on the signal electrode X4 shown by (c) of FIG. 6 indicates ON during a frame period, and is inverted at time t1 when the alternating driving signal is inverted. For the signal electrode X5, only one pixel in a first row is ON, whereas the remaining pixels in second through fifth rows are OFF. Accordingly, the voltage waveform on the signal electrode X5 shown by (d) of FIG. 6 indicates ON corresponding to the pixel in the first row, while indicating OFF corresponding to the pixels in the second through fifth rows. This voltage waveform is inverted at time t1.
Similarly, each voltage waveform on the scanning electrodes Y1 through Y5 is also inverted in response to the inversion of the alternating driving signal. For example, the voltage waveform on the scanning electrode Y1 shown in (e) of FIG. 6 is at a low level at the beginning of the first frame, while attaining a high level at the beginning of the next frame period after time t1.
As a result, a voltage waveform shown in (f) of FIG. 6 is applied to the pixel at the intersection of the signal electrode X4 and the scanning electrode Y1, whereas a voltage waveform shown in (g) of FIG. 6 is applied to the pixel at the intersection of the signal electrode X5 and the scanning electrode Y1.
However, in the case where such crosstalk as mentioned above is present, these voltage waveforms will become as shown in (a) through (g) of FIG. 7.
In this case, a voltage waveform on the scanning electrode Y1 as shown in (e) of FIG. 7 is distorted at time t1 and t2 when the alternating driving signal is inverted. The reason for this will be described in the following in terms of time t1. Before time t1, pixels in the 8 columns of the signal electrodes X1 through X4 and X7 through X10 are ON, whereas pixels in the 2 columns of the signal electrodes X5 and X6 are OFF. In other words, the signal electrodes X1 through X4 and X7 through X10 have a positive potential, whereas the signal electrodes X5 and X6 have a negative potential. Accordingly, positive charges corresponding to 6 dots, the difference in number between the pixels in the ON state and in the OFF state are charged between the scanning electrode Y1 and the signal electrodes. A potential on each of the signal electrodes X1 through X10 is inverted in polarity at time t1. Therefore, these positive charges are discharged through a resistance of the scanning electrode Y1. Thereafter, negative charges corresponding to 6 dots are charged between the scanning electrode Y1 and the signal electrodes through the resistance of the scanning electrode Y1. As a result, the voltage waveform on the scanning electrode Y1 is distorted. Similarly, a voltage waveform on each of the scanning electrodes Y2 through Y5 is also distorted. Since the distortion generation mechanism at time t2 is the same as that at time t1 except for the polarity, description thereof will be omitted.
For example, when the voltage waveform on the scanning electrode Y1 as shown in (e) of FIG. 7 is distorted, a voltage waveform at the pixel at the intersection of the signal electrode X4 and the scanning electrode Y1 as shown in (f) of FIG. 7 is also distorted. Similarly, the voltage waveforms on the other scanning electrodes Y2 through Y5 are also distorted, and the voltage waveforms at the remaining pixels on the signal electrode X4 are also distorted. Therefore, effective voltages applied to the pixels on the signal electrode X4 are reduced, causing reduction in luminance of each pixel on the signal electrode X4.
In addition, a voltage waveform at the pixel at the intersection of the signal electrode X5 and the scanning electrode Y1 as shown in (g) of FIG. 7 is distorted, and an effective voltage applied to the pixel is increased. Similarly, the voltage waveforms at the other pixels on the signal electrode X5 are also distorted, and effective voltages applied to the pixels are increased. As a result, luminance of each pixel on the signal electrode X5 is increased.
Thus, luminance of each pixel on the signal electrode X4 is reduced, whereas luminance of each pixel on the scanning electrode X5 is increased. As a result, vertical stripe lines appear on the display screen.
In order to eliminate such crosstalk, Japanese Laid-Open Publication No. 64-29899 (or see P. Maltese, Eurodisplay Digest, p. 15 (1980)), for example, discloses a method for eliminating distortion of a voltage waveform on each scanning electrode by providing a detection electrode extending in parallel to the scanning electrodes, wherein the detection electrode detects distortion of a voltage waveform induced on each scanning electrode, and applies to every scanning electrode a correction voltage having a polarity opposite to a polarity of the detected distortion so as to eliminate the distortion.
In the case where the above-mentioned method for eliminating crosstalk as disclosed in Japanese Laid-Open Publication No. 64-29899 is applied to the liquid crystal panel shown in FIG. 5, signals for driving the liquid crystal panel are as shown in FIG. 8.
In this case, distortion generated at the detection electrode is detected as distortion of a voltage waveform on any of the scanning electrodes Y1 through Y5. Then, a correction voltage having a polarity opposite to a polarity of the detected distortion is applied to all of the scanning electrodes Y1 through Y5. For example, in the case where distortion generated at the detection electrode is detected as distortion of a voltage waveform on the scanning electrode Y1 as shown in (e) of FIG. 8, a correction voltage having a polarity opposite to a polarity of the detected distortion is applied to the scanning electrodes Y1 through Y5.
In this case, a correction voltage H is added to the voltage waveform on the scanning electrode Y1 as shown in (e) of FIG. 8. In addition, a voltage waveform at the pixel at the intersection of the signal electrode X4 and the scanning electrode Y1 is also corrected as shown in (f) of FIG. 8, whereby an effective voltage applied to the pixel is kept constant. Similarly, voltage waveforms at the remaining pixels on the signal electrode X4 are also corrected, whereby effective voltages applied to the pixels are kept constant.
In addition, a voltage waveform at the pixel at the intersection between the signal electrode X5 and the scanning electrode Y1 is corrected as shown in (g) of FIG. 8, and voltage waveforms at the remaining pixels on the signal electrode X5 are also corrected. Therefore, effective voltages applied to the pixels are kept constant.
As a result, divergence in luminance of each pixel on the signal electrode X4 as well as in luminance of each pixel on the signal electrode X5 is suppressed. Therefore, appearance of vertical stripe lines on the display screen can be prevented.
The above-described conventional method for eliminating crosstalk is effective for such a liquid crystal panel as shown in FIG. 5. However, this method is not effective enough in the case where a single liquid crystal panel is divided into a plurality of display portions and signal electrodes and scanning electrodes are driven on a display portion by display portion basis.
More specifically, a liquid crystal panel is divided into a first display portion 101 and a second display portion 102 as shown in FIG. 9, for example. The first display portion 101 includes signal electrodes X1 through X10 and scanning electrodes Y1 through Y5 located perpendicular to each other for 10.times.5 dot display. Similarly, the second display portion 102 includes signal electrodes x1 through x10 and scanning electrodes y1 through y5 located perpendicular to each other for 10.times.5 dot display. The signal electrodes and the scanning electrodes in the first and second display portions 101 and 102 are driven on a display portion by display portion basis.
A detection electrode is not provided in the first display portion 101. A detection electrode is provided only in the second display portion 102. In such a liquid crystal panel, distortion generated at the detection electrode is detected as distortion in a voltage waveform which is induced on any of the scanning electrodes y1 through y5 by the signal electrodes x1 through x10 in the second display portion 102. Then, a correction voltage having a polarity opposite to a polarity of the detected distortion is applied to all of the scanning electrodes y1 through y5. At this time, the same correction voltage is also applied to all of the scanning electrodes Y1 through Y5 in the first display portion 101.
As can be seen from FIG. 9, display states of the first and second display portions 101 and 102 are opposite to each other. More specifically, ON and OFF states of the pixels in the first display portion 101 are opposite to those of the second display portion 102. In this case, signals for driving the first display portion 101 are as shown in (a) through (e) of FIG. 10.
Although signals for the second display portion 102 are not shown in FIG. 10, distortion in a voltage waveform which is induced on any of the scanning electrodes y1 through y5 in the second display portion 102 is eliminated according to the above-mentioned conventional method for eliminating crosstalk. In other words, distortion generated at the detection electrode is detected as distortion in a voltage waveform which is induced on any of the scanning electrodes y1 through y5. Then, a correction voltage having a polarity opposite to a polarity of the detected distortion is applied to all of the scanning electrodes y1 through y5. Thus, the distortion in the voltage waveforms on the scanning electrodes y1 through y5 can be eliminated.
Since the display states of the first and second display portions 101 and 102 are opposite to each other, distortion in a voltage waveform which is induced by the signal electrodes x1 through x10 in the second display portion 102 will be opposite in polarity to that in a voltage waveform which is induced by the signal electrodes X1 through X10 in the first display portion 101. Accordingly, a correction voltage on correcting a voltage waveform on each of the scanning electrodes y1 through y5 in the second display portion 102 will be opposite in polarity to a voltage which can correct a voltage waveform on each of the scanning electrodes Y1 through Y5 in the first display portion 101.
Accordingly, in the case where a correction voltage h for correcting a voltage waveform on a scanning electrode in the second display portion 102 is added to a voltage waveform on the scanning electrode Y1 in the first display portion 101 as shown in (i) of FIG. 10, a voltage waveform at the pixel at the intersection of the signal electrode X4 and the scanning electrode Y1 as shown in (j) of FIG. 10 changes according to the correction voltage h. However, the effective voltage applied to that pixel is reduced. Similarly, effective voltages applied to the remaining pixels on the signal electrode X4 are also reduced. In addition, a voltage waveform at the pixel at the intersection of the signal electrode X5 and the scanning electrode Y1 as shown in (k) of FIG. 10 also changes according to the correction voltage h. However, the effective voltage applied to the pixel is increased. Similarly, effective voltages applied to the remaining pixels on the signal electrode X5 are also increased.
As a result, vertical stripe lines are prevented from being produced on the display screen in the second display portion 102, while being highly emphasized on the display screen in the first display portion 101.
Alternatively, distortion in a voltage waveform which is induced on any of the scanning electrodes Y1 through Y5 by the signal electrodes X1 through X10 in the first display portion 101 and distortion in a voltage waveform which is induced on any of the scanning electrodes y1 through y5 by the signal electrodes x1 through x10 in the second display portion 102 may be detected individually. In this case, a correction voltage having a polarity opposite to a polarity of the detected distortion is formed separately for each of the first and second display portions 101 and 102. Then, the correction voltages are averaged. The resultant average correction voltage is applied to all of the scanning electrodes in the first and second display portions 101 and 102.
In this case, however, a correction voltage formed for the distortion detected in the first display portion 101 is opposite in polarity to that formed for the distortion detected in the second display portion 102. Therefore, these correction voltages are offset, and an average voltage of the correction voltages will be zero. Accordingly, the voltage waveform on the scanning electrode Y1 in the first display portion 101 will not change before and after the average voltage is added thereto, as shown in (i) and (e) of FIG. 11. As a result, a voltage waveform at the pixel at the intersection of the signal electrode X4 and the scanning electrode Y1 as shown in (j) of FIG. 11 and a voltage waveform at the pixel at the intersection of the signal electrode X5 and the scanning electrode Y1 as shown in (k) of FIG. 11 will not change. Consequently, vertical stripe lines on the display screen will not be eliminated.